1. Field of Invention
The present invention relates to systems for burning gases to provide power or propulsion. In particular, the present invention is directed to systems that can operate in a fully-premixed mode at high pressures, with low pressure losses, and without flashback problems. In specific embodiments, the invention allows for operation in fuel-lean fully-premixed regime and avoids combustion from occurring at stoichiometric fuel-oxidizer contours that result from imperfect mixing.
2. Description of Related Art
The idea of using hydrogen as a fuel for gas turbine combustion is not new, but to make such a system that can operate in a fuel-lean fully-premixed mode at high pressures, with low pressure losses, and without flashback problems has not been solved by the prior art. Operation in the fuel-lean fully-premixed regime avoids combustion from occurring at stoichiometric fuel-oxidizer contours that result from imperfect mixing. Flames at stoichiometric contours have a high flame temperature, thus they produce more NOX (oxides of nitrogen) pollution due to the thermal NOX mechanism.
One design that provides a fully-premixed hydrogen-air flame at high pressure utilizes a water-cooled sintered metal disk as the burner element. An example of such a burner is a ‘McKenna burner,’ manufactured by Holthuis & Associates. The small pore sizes of the porous disk of that burner act as a flashback arrester, which prevents the flame from flashbacking upstream. However, this design requires water cooling to remove the heat from the flame in order to not melt. The water cooling also removes substantial amounts of heat from the flame, making the flame temperature low, which reduces the amount of work the hot gases can provide. Furthermore, this design suffers from high pressure losses making it unsuitable for use in gas turbine applications where pressure losses greater than typically 7% cannot be tolerated.
Another design that attempts to provide a fully-premixed hydrogen-air flame utilizes small hydrogen jets impinging on the main airflow in a ‘cross-flow’ arrangement. In such a design, the main airflow enters a circular duct where multiple (typically 2 to 4) hydrogen gas jets are injected from the wall of the duct radially inwards to the center of the duct. By allowing sufficient distance downstream of the injection point for hydrogen to mix with the air flow before it combusts, a lean premixed system can be realized. However, the problem with this design is that the bulk mixing of the cross-flow hydrogen jets is not complete by the time the mixture burns downstream in a sudden-expansion stabilized combustion zone. Due to the incomplete mixing, this design does not achieve the lowest theoretically permissible levels of NOX emissions since the flame zones sometimes form where there are stoichiometric fuel-air contours that resulted from the incomplete mixing.
Operation in the fuel-lean fully-premixed regime is desirable in order to enable a combustor that produces as little thermal NOX as possible. However, at high pressures, operating a hydrogen-air mixture in a fully-premixed mode has caused thermal meltdown problems in prior art burners due to flashback that causes the flame to anchor upstream of the burner face, thus destroying the burner from the inside.
So the objective is to design a burner that can operate in the fuel-lean fully-premixed mode, yet not suffer from flashback and has a good operability over a wide range of flow conditions, all the while the burner needs to have as little pressure loss as possible and be easy to use and manufacture. However, the requirement of successful operation in a fuel-lean fully-premixed mode does not preclude this combustor design from operating in a fuel-rich fully-premixed mode.